For example, for an air flow measuring device that produces a signal (which may be hereinafter referred to as an intake air amount detection signal) indicating a flow rate of intake air (which may be hereinafter referred to as an intake air amount) drawn into an internal combustion engine, there is conventionally known a device that employs a thermal-type detection method whereby the intake air amount detection signal is produced by use of heat transfer between the device and air. The air flow measuring device includes a housing and an intake air amount sensor described as follows. Specifically, the housing is disposed to project into an intake passage leading to the engine, and defines an internal passage which guides in a part of intake air flowing through the intake passage and through which the intake air passes. The intake air amount sensor projects into the internal passage, and produces the intake air amount detection signal by the heat transfer between the sensor and the intake air passing through the internal passage. The intake air amount sensor includes a high-temperature resistance and a low-temperature resistance described as follows.
Specifically, the high-temperature resistance has its energization controlled in accordance with the intake air amount in the internal passage to increase or decrease its heat generation amount. The low-temperature resistance is disposed, for example, on an upstream side of the high-temperature resistance, and varies its resistance value according to the temperature of intake air (which may be hereinafter referred to as intake temperature) to increase or decrease the energizing amount without the effect of the heat generation of the high-temperature resistance. The high-temperature resistance and the low-temperature resistance constitute a bridge circuit together with a fixed resistance whose resistance value is fixed. This bridge circuit is controlled to produce the intake air amount detection signal based on an electric potential at its predetermined region, for example (see, e.g., JP2003-240620A).
There is a demand on the air flow measuring device for the detection of the intake temperature in addition to the intake air amount. Accordingly, the air flow measuring device in JP2003-240620A detects an applied voltage (V1) to the bridge circuit and a connection point voltage (V3) of the low-temperature resistance (22) and the fixed resistance (24) to enable the detection of the intake temperature by means of the low-temperature resistance (22) (reference numerals in the parentheses are in agreement with the descriptions in JP2003-240620A). However, according to the air flow measuring device in JP2003-240620A, after detecting both the voltages (V1, V3), calculation needs to be further carried out using the voltages (V1, V3). As a result, the circuit configuration and arithmetic processing for detecting the intake temperature are made cumbersome and complicated.
Consequently, separately from the low-temperature resistance included in the bridge circuit, there is proposed such a configuration that a thermistor with a lead wire, for example, is disposed outside the housing as an intake temperature sensor to directly detect the intake temperature in the intake passage (see, e.g., JP2013-029387A). Accordingly, the intake temperature can be detected by detecting one electrical signal, and thus the complication of the circuit configuration and arithmetic processing for detecting the intake temperature can be eliminated. Nevertheless, various measures needs to taken to stably hold the intake temperature sensor outside the housing, or to stabilize the conductive joining between the lead wire and the terminal, so that this configuration is made structurally cumbersome and complicated.